By comparison, 22 per cent cited paperwork, bureaucracy and red tape while fewer than one-in-20 complained about workload and lack of time in the school day, according to Attain, the IAPS magazine.

One head told researchers: “Dealing with the very small number of needy, demanding and unreasonable parents stands out above anything else”.

Another complained of being forced to spend “too much time dealing with minor issues as a result of parental involvement”.

A third head warned of the “unrealistic demands of parents and attitude of them being ‘customers’ buying a product” while another said that “parental interference as opposed to parental interest” was a major problem.

David Hanson, chief executive of IAPS, which represents 600 schools educating children aged around two to 13, said heads were “delighted when parents are actually interested” in school life.

But he said that many schools were being forced to deal with aggressive and hostile attitudes from a minority of parents.

“It is quite troubling at times because they can be not just hostile, but threaten litigation and all sorts of things,” he said.

“It may be that parents are making unsubstantiated claims and threatening to withdraw fees because actually there are financial difficulties in their private life.

“It may be that they just don’t understand what the school is trying to do – they take great exception to the curriculum – or maybe they have expectations of their own child’s performance and their future senior school of choice that are just not realistic.”

In a further question, heads were asked whether any factors had changed since they first joined the profession which would “no longer encourage you to work in private schools”.

Some 32 per cent cited paperwork, 26 per cent named Government interference and 21 per cent identified the “involvement of parents”.

Julie Robinson, a former headmistress and IAPS education director, said: “There is a fine line between being supportive and being overly pushy.

“A huge chunk of the head’s role is to manage parents’ expectations and it is not surprising at all that they have brought this up as a huge, time-consuming part of their work.”